1. Field of the Invention
This invention generally relates to an arrangement for, and a method of, detecting the presence or absence of persons or things in an area, such as an entranceway, window or security zone, both outdoor and indoor, despite such environmental variations as ambient light and temperature fluctuations and, more particularly, to fail-safe obstruction detectors especially useful in systems for controlling garage doors, elevator doors and the like, and to security systems for controlling access to a secure area.
2. Description of the Related Art
An obstruction detector for an automatic garage or elevator door control system prevents personal injury and/or property damage to a person or thing caught in the closing door, as well as preventing damage to the door itself and the various drive components that close the door. It has been proposed to use a light transmitter to transmit a light beam, either along a direct or a folded path, across a door opening for detection by a light receiver. Unobstructed receipt of the light beam indicates that the entranceway is free of the object. However, failure to receive the light beam indicates that the entranceway has an obstruction. This failure is electronically processed by a drive controller to either stop or reverse a closing door.
Analogously, it is known in security systems to transmit a light beam across a window or security zone. Interruption of the beam triggers an alarm, thereby summoning law enforcement personnel.
Although the known obstruction detectors and security systems are generally satisfactory for their intended purpose, experience has shown that environmental variations can adversely affect system operation. For example, in garage door applications, ambient light varies and, in some cases, widely, depending, for example, on the level of sunlight, passing clouds, weather conditions, the presence of headlights from passing vehicles, the activation of outdoor and/or indoor garage lights, etc. The receiver is not only exposed to the transmitted light beam, but is also exposed to such ambient light which, in some cases, at least for a limited time, has a level of intensity sufficient to "blind" the receiver and "confuse" the drive controller. Similar considerations apply for the ambient temperature fluctuations, wherein the receiver responds differently and in a non-linear manner at different temperature levels.
Also, some known obstruction detectors, especially in garage and elevator door control systems, are defeated by insuring that the light beam path would not be interrupted by a person entering the entranceway. For example, a reflector could be closely positioned adjacent the light transmitter and receiver. Whether intentional or not, such easily defeated systems do not serve their originally intended function.